Minimize allocations and avoid rebuilding weighted list when retrieving potential spawns

[embeddedt]

The current implementation of EventHooks.getPotentialSpawns is rather inefficient - it fires the event, then constructs an entirely new weighted random list from the event's results, even if no mods changed the potential spawns. This is not very efficient and leads to a lot of allocation overhead beyond the vanilla code, as shown here:

This PR solves the issue by making a few changes:

• LevelEvent.PotentialSpawns now allocates its backing list lazily, rather than in the constructor.
• EventHooks now takes advantage of that change to reuse the existing weighted list from vanilla if no mods modify potential spawns.

With these changes, the only allocation overhead left is that of allocating the LevelEvent.PotentialSpawns object itself. Unfortunately, that still shows up in the allocation profiler, but solving it is not really possible unless we move to pooling hot events like this.

[neoforged-pr-publishing]

• Publish PR to GitHub Packages

Last commit published: cdc8ec88b828a4a3c035f88ede142f9f7b63c49d.

PR Publishing

The artifacts published by this PR:

• 📦 net.neoforged:neoforge:20.4.212-pr-732-potentialspawns-alloc
• 📦 net.neoforged:testframework:20.4.212-pr-732-potentialspawns-alloc

Repository Declaration

In order to use the artifacts published by the PR, add the following repository to your buildscript:

repositories {
    maven {
        name 'Maven for PR #732' // https://github.com/neoforged/NeoForge/pull/732
        url 'https://prmaven.neoforged.net/NeoForge/pr732'
        content {
            includeModule('net.neoforged', 'neoforge')
            includeModule('net.neoforged', 'testframework')
        }
    }
}

MDK installation

In order to setup a MDK using the latest PR version, run the following commands in a terminal.
The script works on both *nix and Windows as long as you have the JDK bin folder on the path.
The script will clone the MDK in a folder named NeoForge-pr732.
On Powershell you will need to remove the -L flag from the curl invocation.

mkdir NeoForge-pr732
cd NeoForge-pr732
curl -L https://prmaven.neoforged.net/NeoForge/pr732/net/neoforged/neoforge/20.4.212-pr-732-potentialspawns-alloc/mdk-pr732.zip -o mdk.zip
jar xf mdk.zip
rm mdk.zip || del mdk.zip

To test a production environment, you can download the installer from here.